2.2 Enunciado del problema
3.5.1 Para procesamiento y análisis de datos
cu 3 0
splotch parent colony NZW/5p“ F, DBA/Sp^" F,
I I E x e n c e p h a l y ■ H S p i n a b ifid a
4.4 Discussion
Position o f closure 2 varies between mouse strains
The position of closure 2 is variable between but not within inbred strains. This was first established by Juriloff et al. (1991) in a variety of normal strains and has been shown to be true of the strains studied in Section 4.2.1. The nature of closure 2 not only varies in its position along the rostrocaudal axis but also in the length of the initial contact between the neural folds, and in its timing in relation to closure 3. The contact can either be as a discrete point or as a band. This study found that closure 2 occurs in one of 3 positions: at the forebrain/midbrain boundary (‘intermediate’), or rostral (‘low’) or caudal (‘high’) to this boundary. A point contact appears only to occur when closure
2 is at the forebrain/midbrain boundary, an observation made both in the present study
and in the initial observations of Juriloff et al. (1991).
Work on human abortuses and neonates with NTD has provided strong evidence to suggest that multi-site closure of the neural tube also occurs in the human embryo (Van Allen et a l 1993; Seller, 1995). It is therefore possible that the variation seen in the position of closure 2 in the mouse is true of the human embryo, in which case, the idea
that variation in the site of closure 2 confers susceptibility or resistance to cranial NTD may also apply to humans.
There is strong evidence that ethnic groups vary in their susceptibility to NTD (reviewed in Section 1.3.2), raising the possibility that human embryos of different racial origins may vary in the position of closure 2. A study by Van Allen et a l (1993) suggests that failure of closure 2 is involved in the generation of NTD in Walter- Warburg syndrome and maternal hyperthermia whereas other subpopulations of cranial NTD appear to have an intact closure 2 (e.g. Meckel-Gruber syndrome). An alternative
explanation is that closure 2 may still occur, but that it is rostrally situated in embryos
destined to develop NTD (e.g. in Walter-Warburg syndrome) so that closure is unable to extend into the midbrain region. Thus, the striking correlation between the location of NTD along the body axis and the proposed closure sites and neuropores in the embryo can be taken as evidence that variation in genetic background between ethnic groups may affect susceptibility to NTD in the same way as in mouse (Van Allen et al. 1993).
Work in Chapter 3 demonstrated that embryos of the splotch background have a rostrally positioned closure 2 and it was postulated that this may exacerbate the cranial
NTD resulting from the splotch mutation. This hypothesis was tested by backcrossing the 5/7^^ mutation onto the DBA/2 background in which a ‘high’ position of closure 2 is
seen, caudal to the forebrain-midbrain boundary. As a control, the 5/?^^mutation was backcrossed onto the NZW background, which exhibits a ‘low’ position of closure 2, as on the splotch background. It was predicted that in the NZW backcross, the position of closure 2 would not alter and that the incidence of exencephaly would remain the same.
Hence, the NZW backcross provides a control for alteration of genetic background, without alteration of the position of closure 2.
Position o f closure 2 becomes more caudal in the DBA/2 backcross but remains
rostral in the NZW backcross
The intermediate position of closure 2 between that of splotch and DBA/2 (i.e. unlike either parent) in splotch x DBA/2 F, embryos, together with the gradual shift in the position of closure 2 in subsequent backcross generations, suggests that multiple genes
control the position of closure 2. A single controlling gene would be expected to yield a closure 2 position resembling one of the parental strains. Moreover, four backcross generations were required to convert the closure 2 position from low (splotch-Mke) to
splotch alleles by DBA/2 alleles at a number of controlling loci. According to this model, the position of closure 2 is the resultant of the additive effects of two types of
allele.
The number of backcross generations required to reach this full DBA-like phenotype reflects the number of loci controlling the position of closure 2. Figure 4.*? shows a theoretical calculation of the number of loci involved in determining the position of closure 2. This model calculates that 3-5 loci are involved on the basis that splotch is entirely homozygous for all alleles. However, if splotch was heterozygous at some of these loci, more genes would be involved in determining the position of closure 2 (see
Figure 4.6)
In the SELH strain, in which approximately 17% of embryos develop exencephaly, all embryos have a genetic liability to exencephaly in that they lack closure 2 (MacDonald et a l 1989). The number of involved loci in determining the SELH phenotype (lack of closure 2) has been studied by an crossing the SELH and ICR strains and subsequently backcrossing to SELH. This study predicts that depending on the number of loci involved, a certain number of sires will transmit the SELH-like incidence of exencephaly:
1 locus: 1/2 sires are SELH-like 2 loci: 1/4 sires are SELH-like 3 loci: 1/8 sires are SELH-like.
The study concluded that 2 or 3 loci were involved in determining exencephaly (i.e. lack of closure 2). (Juriloff et a l 1989).